The Cubit

The Egyptian ordinary cubit was divided into 24 fingers, and the Egyptian foot was 16 fingers, or 2/3 of a cubit. The cubit was the length from the tip of the finger to the elbow; thus it was essentially half a yard. Many ancient terms for this unit mean "elbow".

Surviving Egyptian rulers use the royal cubit which was an ordinary cubit plus four fingers, and this royal cubit was the basis of ancient Egyptian architecture and other measurements. A natural cubit is 6/7 of the royal cubit. The Greeks and Romans took up the natural cubit, while other ancient and Mesopotamian cultures continued the use of the royal cubit for sacred architecture.

Given the imprecision of ancient meauring tools, it's possible to only imprecisely compare ancient Egyptian measures to contemporary ones. The surviving royal cubit rulers are 526 mm, plus or minus 3 mm, so an ordinary cubit would be 450.8 mm. The customary American foot is 457 mm. If metric inch (25 mm) was used instead of the international inch (25.4 mm), the ancient Egyptian ordinary cubit would be exactly 18 metric inches.

This is a coincidence. The ancient Egyptian system of measurement was the basis of the Greek system of measurement, which was the basis of the Roman system of measurement, which was the basis for contemporary customary measurement. But the system shifted over time. The Romans emphasized the inch ("uncia") as a unit of measure, and the Roman inch and the Roman cubit were a few miliimeters shorter than the contemporary American standard. The word "cubit" itself comes from the Roman word for the unit, "cubitus."

Effectively, a cubit is a measure of 18 inches, a foot and a half, or half a yard, which in ancient times was closer to 450 mm.